Process and device for data transmission in a cellular radio system

ABSTRACT

In order to transmit data packets (DP) in a cell (Z) between a base station (BTS) and a mobile station (MS) of a cellular radio system corresponding to a time division multiplex method in time slots (TN) of a time division multiplex frame (T), after in each case one prescribed time period the transmission of the data (DP) is reduced during in each case one time interval (P). During these time intervals (P), base stations (BTS) in adjacent cells (Z) are radio-monitored.

[0001] The invention relates to a process and an arrangement fortransmitting data in a cellular radio system. In addition, the inventionrelates to a corresponding subscriber station and central station.

[0002] In a mobile radio system, an area to be supplied is usuallydivided into a large number of radio cells. In these radio cells thereis provision in each case of base stations via which the connectionsbetween a public telephone network and the subscriber stations of theindividual subscribers, for example mobile stations, in the respectivecell can be established. Such a mobile radio system is, for example, theGSM (Global System for Mobile Communication) standardized by theEuropean Telecommunications Standards Institute (ETSI). Such a system isdescribed, for example, in a brochure “D 900 Mobile CommunicationSystem” SYD from Siemens AG, 1992. A similar system is the DCS 1800/PCN.

[0003] In the mobile radio station illustrated in FIG. 5, a radio regionis divided up, in accordance with the GSM standard, into a plurality ofcells Z, of which only the cells Z1 to Z7 are illustrated in FIG. 5. Ina cell Z the telecommunications service from and to a mobile station MSis made available by means of a base station BTS (BTS=Base TransceiverStation), which is part of a central station. In FIG. 6, only the basestations BTS1 to BTS3 assigned to the cells Z1 to Z3 are illustrated.One or more base stations BTS are connected to a base control unit BTSC(BTSC=Base Station Controller), of which only the base control unitsBTSC1 and BTSC2 are illustrated in FIG. 5. The base control units BTSCperform the local functions of call switching, monitoring andmaintenance. They comprise, in particular, control units BCE (BaseStation Control Equipment). A plurality of base control units BTSC areconnected to a switching device SSS (SSS=Switching Subsystem) which isitself connected to the public network PSTN (PSTN=Public SwitchedTelephone Network) which can be designed as an ISDN network, as a mobileradio network or as some other telephone or data network.

[0004] If a subscriber wishes to communicate with another subscriber bymeans of a mobile station MS in the cell Z1, a prescribed protocol isprocessed via the base station BTS1 with the base control unit BTSC1,which protocol is defined, for example, in the abovementioned GSMstandard. When the connection between the mobile station MS and the basecontrol unit BTSC1 has been established, a connection is established tothe public network PSTN via the switching device SSS. From said publicnetwork PSTN, the other subscriber, who may be in turn provided with amobile station, is then reached.

[0005] In the GSM, the signals are usually transmitted using theso-called TDMA (Time Division Multiple Access) method in a frequency andtime division multiple access. In this method, the data, which may alsocomprise digitalized voice, are transmitted via time slots of a TDMAframe.

[0006] During the transmission, the mobile station observes the radiofield conditions of the adjacent base stations so that, if appropriate,a handover to another base station can be performed on the basis ofthese measurements.

[0007] In order to obtain relatively high data rates during transmissionbetween the mobile station and the network, a plurality of time slots ofthe TDMA frame may be used for the same connection. However, thisshortens the time period in which a mobile station can observe theadjacent base stations. If, for example, all the time slots of a TDMAframe are used for a connection, it may be even be completely impossibleto observe the adjacent base stations. In order, nevertheless, to ensureobservation takes place, it would be conceivable to provide a secondreceiver to monitor the adjacent base stations in the mobile station,which receiver would be used, in particular, to observe the adjacentbase stations. However, such a solution has the disadvantage that itrequires a relatively large amount of expenditure and additional costsand, furthermore, increases the volume of the mobile station.

[0008] EP 662 778 A2 discloses the Asynchronous Transfer Mode (ATM) fortransmitting permanently prescribed data packets by mobile radio. Themonitoring of neighboring cells in a cellular system is not addressed inthis known process of transmission.

[0009] Furthermore, WO 91-2436 A1 describes the provision in each basestation of a cellular mobile radio system of devices which detect thetemporary pauses in the transmission between the base station and themobile station and bring about signaling operations with the mobilestation during these pauses.

[0010] The invention is based on the objective of specifying a processand an arrangement by means of which a connection to a plurality of timeslots is established in a simple and cost-effective way and,nevertheless, the use of a second receiver in the mobile station isavoided.

[0011] According to the invention, the object is achieved in the processby means of the measures specified in patent claim 1. An arrangement, acentral station and a subscriber station for carrying out the processare specified in patent claims 14, 16 and 17. Developments of theinvention emerge from the subclaims.

[0012] An exemplary embodiment of the invention is explained in moredetail below with reference to drawings, in which:

[0013]FIG. 1 shows a schematic illustration of data to be transmitted,

[0014]FIG. 2 shows the structure of a time slot frame,

[0015]FIG. 3 shows the structure of a frame formed from a plurality oftime slot frames,

[0016]FIG. 4 shows a three-dimensional, schematic illustration of datatransmitted simultaneously over a plurality of time slots of a time slotframe, the data rate being in each case briefly reduced during thetransmission, and

[0017]FIG. 5 shows a block diagram of a mobile radio system.

[0018] The illustration in FIG. 1 shows schematically a data packet DPto be transmitted on the air interface between the mobile station andthe base station. This data packet DP is divided up into data packetsegments DPS1 to DPSn and which are separated from one another by meansof a time interval P of, for example, 1 s, while the data transmissionrate to them is reduced. These data packet segments DPS are transmittedsimultaneously by means of a corresponding transmission protocol, forexample the so-called modified RLP (Radio Link Protocol), via aplurality, for example up to eight, of time slots of a TDMA frame on onefrequency.

[0019]FIG. 2 illustrates the structure of a GSM time slot frame (TDMAframe) T with eight time slots TN0 to TN7, one time slot frame having aduration of 4.615 ms. Each time slot has a duration of approximately0.577 ms or 156.25 bits. The physical contents of a time slot arereferred to as a burst. There are four different types of burst in thissystem:

[0020] 1. Normal burst: This burst is used to transmit data, voice ormonitoring information.

[0021] 2. Frequency correction burst: This burst is used for frequencysynchronization of the mobile station.

[0022] 3. Synchronization burst: This burst is used for framesynchronization of the mobile station.

[0023] 4. Access burst: This burst is used for the initial access of themobile station and at a handover of the mobile station.

[0024] A normal burst TN with a duration of 0.577 ms is illustrated inan enlarged form in FIG. 2. It contains 114 encrypted or unencryptedinformation bits, depending on whether an encryption function isswitched on or off. These bursts are divided up into two half bursts D1and D2 with 57 bits each. In the center of the normal burst there is a26-bit long training sequence TSC which serves to dimension the channeland/or estimate the channel impulse response of the transmissionchannel. On each side of the training sequence TSC there is a controlbit CB which indicates whether the normal burst contains data ormonitoring information. A normal burst begins and ends with threedefined additional bits (Tail Bits) TB in each case. A normal burst endswith a protection time (Guard Period) GP of 8.25 bits.

[0025] The traffic data channels in the GSM are arranged in a frame TFconsisting of 26 time slot frames (26 frame/multiframe) such as isillustrated in FIG. 3. In twelve successive time slot frames T, namelythe time slot frames 0 to 11 and time slot frames 13 to 24, data,digitalized voice or monitoring information are transmitted. The timeslot frame 12 is used to transmit control information A such asparameters for adaptively regulating the performance of thecorresponding mobile station, frequencies of the neighboring cells, etc.and cell-specific information. No data is transmitted in the time slotframe 25.

[0026] The frame TF, which is illustrated in FIG. 3, corresponds to aduration of 120 ms. Useful information can thus be transmitted in 24 ofthe 26 time slot frames T. Since 114 bits (normal burst) can betransmitted in each time slot TN, 114×24=2736 information bits aretherefore transmitted during a 120 ms-long frame. This corresponds to agross data rate of 22.8 kbit/s.

[0027] From 51 such frames TF, a superframe with a duration of 6.12 s isformed for the transmission of useful information, and 2048 suchsuperframes form a hyperframe with a duration of 3 h 28 m 53 s 760 s.

[0028] If, for illustrative reasons, the frames T are not illustratednext to one another but rather graphically one behind the other, theillustration according to FIG. 4 is obtained. Here, the time slots TN ofthe time slot frames T are illustrated on a time axis n1 and thesuccessive time slot frames T are illustrated one behind the other onanother time axis n2.

[0029] If the data of the data packet segments DPS are not transmittedover, in each case, one time slot of a time slot frame T but rather overa plurality of time slots TN of a time slot frame T on the samefrequency, the data transmission rate is correspondingly increased. Inthe illustration it is assumed by way of example that the transmissiontakes place simultaneously over the five time slots TN0 to TN3, which isillustrated by hatching from the bottom left to the top right. In FIG.4, only the direction from the base station BTS to the mobile station MS(downlink) is illustrated. The transmissions in the two directions areoffset with respect to one another by three time slots TN. If in eachcase only one time slot TN is used for a connection, there remainssufficient time between them to monitor the base stations of theneighboring cells. However, if the transmission of data takes place inparallel simultaneously in a plurality of time slots TN, there remainsno further time for observing the neighboring cells.

[0030] If the transmission duration of each data packet segment DPS witha high transmission rate is limited in time, the time intervals Pillustrated in FIG. 1 between the transmissions of the successive datapacket segments DPS are used for observing the neighboring cells. Whentransmitting data to the mobile station MS, it is, in any case,expedient to set up and release the connection for the purpose ofrouting and in order to avoid handovers during the data transmission.

[0031] The time period of the data packet segments DPS illustrated inFIG. 1 is for example 30 s. This ensures that neighboring-cellobservation takes place at the latest after, for example, 30 s in eachcase. This corresponds to the time period after which, in the case ofGSM, a cell selection (cell reselection) is carried out if the mobilestation MS is in a battery saving mode or in a no-load mode. Thebehavior and the accessibility of the mobile station MS are thus notadversely affected. In addition, the net data rate is reduced onlyslightly. However, it is significant, in particular, that with theprocess indicated a second receiver is not necessary in the mobilestation MS.

[0032] Instead of the monitoring of all the adjacent base stations BTSafter, in each case, one data packet segment DPS with the time period of30 s, the adjacent base stations BTS can also successively be monitoredafter, in each case, a relatively short time period in the time intervalP.

[0033] Then, too, it is ensured that all the adjacent base stations BTSare radio-monitored after, in each case, a prescribed time period.

[0034] In the illustration in FIG. 4, the data transmission rate isbriefly reduced during the ongoing connection. The time slot frames inwhich this takes place are indicated by hatching from left bottom to topright. For this purpose, the number of time slots TN which are used forthe transmission of data is briefly reduced. For example, thetransmission of data during one or a small number of time slot frames Tis reduced. As a result, there is sufficient time available to acquirethe measured values.

[0035] In the illustration in FIG. 4, for example five time slots TN areprovided in each case for normal transmission of data. If theneighboring cells are to be monitored, the number of time slots TNduring a time interval is reduced to, for example, two so thattransmission continues only to the time slots TN0 and TN1. The number oftime slots TN used for the transmission can be reduced here according toa permanently agreed schema, the agreed schema then being transmitted,for example, by means of the signaling system. The reduction can also beperformed, in each case, by the mobile station MS depending on therequirements. In this case, a dependability protocol may be requiredwhich detects the loss of the data which have not been received owing tothe monitoring of a neighboring cell and, if appropriate, requests theirrepetition. The data are therefore usually transmitted in data blockswhich themselves can be distributed again over a plurality of bursts(for example by means of so-called interleaving).

[0036] The time slots selected for monitoring the neighboring cells arepreferably ones which can be used for monitoring bursts of a block. As aresult, the number of disrupted blocks is reduced. In the permanentlyagreed schema, such preventive measures are not necessary since thetransmit side and the receive side each know a priori at which times themobile station MS is not ready to receive in specific time slots TN.

[0037] The transmission can also be combined with a dynamic loaddistribution in which the data blocks to be transmitted are distributedbetween the time slots TN associated with the various channels, in sucha way that more data blocks are transmitted over channels with a hightransmission quality, few repetitions and thus a high data throughputrate than over channels with a low transmission quality and a small datathroughput rate. A channel whose time slots TN are used for monitoringthe neighboring cells has, in the case of the agreed schema, a datathroughput rate which is known a priori and which, in the case ofmonitoring, appears in accordance with requirements as a channel with apoor transmission rate, and thus likewise low data throughput rate.

[0038] The load distribution mechanism, which may be for example aconnection protocol modified for multiple access transmissions,distributes, in accordance with the possible data throughput rate, thedata to be transmitted, between the traffic channels associated with aconnection.

1. A process for transmitting data (DP) in a cellular radio system between a base station (BTS), provided in a cell (Z), and a mobile station (MS), in accordance with a time division multiplex method in time slots (TN) of a time division multiplex frame (T), and in which during the transmission of the data (DP) the base stations (BTS) in adjacent cells are radio-monitored, characterized in that after, in each case, a prescribed time period during the transmission of data (DP), a prescribed time interval (P) is provided during which the transmission takes place over a reduced number of time slots (TN), in that during this time interval (P), in each case, the base stations (BTS) of adjacent cells (Z) are radio-monitored, and in that after this time interval (P) has expired, the transmission of the data is continued again at the original data rate.
 2. The process as claimed in claim 1 , wherein the number of time slots (TN), during which a reduced transmission of the data takes place, is smaller than the number of time slots (TN) lying between the transmission from and to the mobile stations (MS).
 3. The process as claimed in claim 1 or claim 2 , characterized in that during each time interval (P) all the base stations (BTS) are radio-monitored in the adjacent cells (Z).
 4. The process as claimed in one of claims 1 or claim 2 , characterized in that during successive time intervals (P) the base stations (BTS) in the adjacent cells (Z) are successively radio-monitored.
 5. The process as claimed in one of the preceding claims, characterized in that the number of time slots (TN) which are used for a transmission of the data at the high data rate is defined according to a schema agreed between the base station and the mobile station.
 6. The process as claimed in claim 5 , characterized in that the agreed schema is transmitted by means of the signaling system.
 7. The process as claimed in one of the preceding claims, characterized in that the reduction of the time slots is performed by the mobile station (MS) depending on the requirements during the time interval.
 8. An arrangement for carrying out the process as claimed in claim 1 , characterized in that devices are provided which, after in each case a prescribed time period during the transmission of the data (DP), provide a prescribed time interval (P) during which the transmission takes place by means of a reduced number of time slots (TN) which, during this time interval (P), radio-monitor in each case the base stations (BTS) of adjacent cells (Z) and which, after this time interval (P) has expired, continue the transmission of the data again at the original data rate.
 9. A central station for carrying out the process as claimed in claim 1 , characterized in that it contains devices which, after in each case a prescribed time period during the transmission of the data (DP), provide a prescribed time interval (P) during which the transmission takes place by means of a reduced number of time slots (TN) and which, after this time interval (P) has expired, continue the transmission of the data again at the original data rate.
 10. A subscriber station for carrying out the process as claimed in claim 1 , characterized in that it contains devices which radio-monitor the base stations (BTS) in the adjacent cells (Z) during the time intervals (P) during which the data are transmitted at a reduced data rate. 